1. Field of the Invention
This present invention relates generally to polyimides. It relates in particular to polyimide foams and a process for the preparation of polyimide foams for widespread applications in the aerospace, marine, automotive and building construction industries.
2. Description of the Related Art
Polyimide foams have a number of beneficial attributes for many applications. As a result, they are employed in joining metals to metals or metals to composite structures; as structural foam, having increased structural stiffness without large weight increases; and as low density insulation for thermal and acoustic applications.
Methods for making polyimide foams as disclosed in U.S. Pat. Nos. 5,298,531; 5,122,546; 5,077,318; and 4,900,761 utilize solutions of diamines and dianhydrides or dianhydride derivatives in a low molecular weight alkyl alcohol solvent. Polyimide precursor solutions and powders therefrom are then processed into foams through the expulsion of water and alcohol during a thermal imidization process. Unfortunately, foams prepared by these methods are not available in a wide range of densities, especially very low densities, along with the desired combination of mechanical properties and flame resistance. Moreover, thermal energy must be applied to the precursors to produce the foam, thereby limiting the applicability of the processes.
Polyimide foaming processes as disclosed in U.S. Pat. Nos. 4,738,990; 6,057,379; and 6,133,330 all employ powder precursors. As a result, these processes do not present the widest possible range of applicability, howsoever efficacious they might be.
Polyimide foaming processes as disclosed in U.S. Pat. Nos. 6,057,379 and 4,946,873, as well as U.S. Patent Application Publication 2003/0065044A1, all require the application of microwave radiation to initiate the foaming process. Such a requirement presents a significant limitation on the applicability of these processes.